The prior art, for example the document US 2010/0245777 A1, discloses lighting devices for projection applications which comprise a phosphor wheel having one or more phosphors. In this case, said lighting devices comprise a pump light source, which excites the phosphor to emit light having a different wavelength than the pump light wavelength (wavelength conversion of the pump light by means of phosphor). The phosphors are usually arranged successively in the direction of rotation of the phosphor wheel, such that the light (conversion light) emitted by the respective phosphor is generated temporally sequentially and fed to the imaging system.
Lasers, for example laser diodes, are preferably used as the pump light source. In this case, the technology is also known by the designation LARP (“Laser Activated Remote Phosphor”). In order to distribute the laser power over a larger area on average over time and thus to reduce thermal quenching of the phosphor, inter alia, the at least one phosphor is usually applied to a wheel that rotates during operation. A power loss that increases with the pump power and is based for the most part on the difference in energy of the photons between the absorbed radiation and the emitted radiation (Stokes shift) is deposited in said phosphor wheel. Since the pump light is usually in the short-wave spectral range, for example in the blue range, the power loss in the case of phosphors which emit in the red spectral range (red phosphors) is higher than that in the case of phosphors which emit in the green spectral range (green phosphors). This power loss leads to heating of the phosphor wheel. The heat is dissipated by surface radiation, by convection of the ambient medium (e.g. air) and by thermal conduction, e.g. via the rotation spindle, such that a stable operating temperature is established. The conversion efficiency of many phosphors, e.g. nitridic phosphors for the red spectral range (nitridic red phosphors), decreases as the operating temperature rises. The increase in the pump power or power density that is required for higher output powers (light power) results in a higher heat loss or concentration of heat. This can be partly compensated for by the diameter of the phosphor wheel being increased, since a larger area is then available for the dissipation of heat (original diameter of 33 mm, then 40 mm, now 55 mm, in future up to 85 mm). Limits are imposed on this power scaling concept owing to the demands in respect of space requirement and also in respect of the power of the wheel motor.